In recent years, higher zoom ratio, higher performance and smaller size are demanded for imaging optical systems, including a video camera and a digital still camera. A zoom lens that is proposed to meet such demands is a zoom lens comprising, in order from an object: a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having positive refractive power; and a fourth lens group having positive refractive power, wherein zooming is performed by moving each lens group (e.g. see Japanese Laid-Open Patent Publication No. 2008-152288(A)).